O'Carolan composed both songs and instrumental harp music. Except for one song with an English text, all of his songs are in Irish. Most of his songs were dedicated to and about specific individuals. Many songs do not survive whole; what lyrics survive have only been published in part. His lyrics are rarely learned, whereas many of his tunes are widely performed and appreciated.
O'Carolan's musical style shows a mix of traditional and classical elements. He typically composed the tune first—as he rode from place to place—then added words later. Many of the tunes attributed to O'Carolan are older traditional melodies that he improved or lengthened. He wrote many "planxties" (tributes) in honor of some person. It is said that weddings and funerals were often delayed until he could arrive to perform.